1. Field of the Invention
The present invention relates to a DC-DC converter, and particularly to a DC-DC converter which enables reduction of switching loss and loss due to the recovery current of a diode.
2. Description of the Related Art
A DC-DC converter, which converts an inputted DC voltage into a desired stable DC voltage based on the switching control by a semiconductor device, has advantages such as a high efficiency, easy reduction in weight, and the like, and therefore is extensively and indispensably used in power supply for various electronic equipments, in inverter technology based control of electric machines, and in a circuit for lighting a discharge lamp.
FIG. 11 is a circuitry of a typical step-down DC-DC converter. Referring to FIG. 11, a DC-DC converter 200 includes a field effect transistor Q1 as a main switching element, a flywheel diode D3, a choke coil L1, an output capacitor C5, and a control circuit 202, wherein a voltage Vi is a DC power source, and a resistor R1 is a load. A capacitance C1 is a junction capacitance between the drain and source of the field effect transistor Q1, and a diode D1 is a parasitic diode of the field effect transistor Q1.
The DC power source Vi has its positive terminal connected to the drain terminal of the field effect transistor Q1 and has it negative terminal grounded. The source terminal of the field effect transistor Q1 is connected to the cathode terminal of the flywheel diode D3 and also to one terminal of the choke coil L1 which has its other terminal connected to one terminal of the output capacitor C5. The other terminal of the output capacitor C5 and the anode terminal of the flywheel diode D3 are grounded. The control circuit 202 is connected via its detection terminal to the other terminal (toward the load R1) of the choke coil L1 and via its output terminal to the gate terminal of the field effect transistor Q1.
The operation of the DC-DC converter 200 will be explained. Under a steady state condition with the field effect transistor Q1 set turned-off, when the field effect transistor Q1 is turned on, a current flows from the DC power source Vi to the choke coil L1 via the field effect transistor Q1, and a voltage at the other terminal (toward the load R1) of the choke coil L1 is smoothed by the output capacitor C5 and then applied to the load R1. While the field effect transistor Q1 stays turned-on, energy is stored up in the choke coil L1 according to the current. Then, when the field effect transistor Q1 is turned off, electromotive force is generated across the both terminals of the choke coil L1, and the current maintained by the electromotive force commutates to flow through the flywheel diode D3, whereby the energy stored up during the turn-on period of the field effect transistor Q1 is supplied to the load R1.
With repletion of the operation described above, a voltage according to the duty ratio (on-time/on-time+off-time) of the field effect transistor Q1 is outputted across the both terminals of the load R1. In order to keep the output voltage constant irrespective of variations of the input voltage Vi and the load R1, the control circuit 202 performs pulse width modulation (PWM) control, in which the duty ratio of the field effect transistor Q1 is modulated according to a detected output voltage.
In the DC-DC converter 200 described above, due to the junction capacitance C1 formed between the drain and source terminals of the field effect transistor Q1 and also due to wiring-related parasitic inductances, a transitional period, at which a non-zero voltage across the drain and source terminals and a non-zero drain current are concurrently present, arises at the moment when the field-effect transistor Q1 turns on or turns off, and thereby a switching loss is caused. Since the switching loss becomes larger with increase of a frequency for performing on-off control, a serious problem is involved when reduction of the dimension and weight of an apparatus is sought to be achieved by increasing the on-off control frequency so as to reduce the inductance of a choke coil and the capacitance of an output capacitor. Further, there is another problem that when the field effect transistor Q1 turns off thereby reverse-biasing the flywheel diode D3, a large recovery current is caused to flow from the cathode to the anode at the reverse recovery time resulting in causing a heavy loss.
Under the circumstances described above, what is called a “soft switching technique” is conventionally applied which utilizes resonance thereby reducing switching loss or loss attributable to the recovery current. For example, Japanese Patent Application Laid-Open No. 2003-189602 discloses a DC-DC converter as shown in FIG. 12, in which a resonant circuit uses the junction capacitance of a switching element and a rectifying element in order to deal with an extensive range of an input and output voltage variation.
Referring to FIG. 12, in a DC-DC converter 300, the source terminal of a field effect transistor Q1 is connected via a resonant coil L2 to the junction of a flywheel diode D3 and a choke coil L1, and a series circuit composed of the resonant coil L2 and the flywheel diode D3 is connected in parallel to a series circuit composed of a resonant capacitor C4 and a field effect transistor Q2. A diode D6 and a capacitor C6 are connected in parallel across the drain and source terminals of the field effect transistor Q2, a diode D1 and a capacitor C1 are connected in parallel across the drain and source terminals of the field effect transistor Q1, and a diode D5 is connected in parallel to a series circuit composed of the field effect transistor Q1 and the resonant coil L2. In the DC-DC converter 300 described above, the switching loss and noises can be reduced due to a zero-voltage switching realized by the resonance between the resonant coil L2 and the capacitors C1 and C6 (parallel capacitors) of the field effect transistors Q1 and Q2.
However, the DC-DC converter 300 requires a plurality (two in the figure) of field effect transistors Q1 and Q2, and a resonant capacitor C4, which invites an increase in component cost and also in structural dimension. And, while the switching loss of the field effect transistor Q1 as a main switching element and the loss of the flywheel diode D3 are reduced, losses are otherwise incurred at the field effect transistor Q2 as an auxiliary switching element and at the resonant coil L2, which results in that the operating efficiency of the DC-DC converter 300 is not tangibly improved as a whole.